This invention relates to a combustible gas diode sensor including a SiC semiconductor substrate on top of which an AlN layer and a catalytic metal xe2x80x9cgatexe2x80x9d electrode are deposited. The resulting devices can be operated in either a D.C. forward conduction or an A.C. reverse bias mode.
MOS combustible gas sensors operate by catalytic oxidation of combustible gases at the xe2x80x9cgatexe2x80x9d. Substantial efforts have been expended in recent years towards the development of combustible gas sensors using semiconductor MOS technology.
Generally the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by a SiO2 insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gases. As a result of catalytic redox reactions on the gate surface, certain atomic and molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/semiconductor interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent A.C. admittance characteristic of the device along the voltage axis.
In order to be sensitive to combustibles other then H2, the catalytic gate and, therefore, the device have to be operated at temperatures above 400xc2x0 C., requiring the use of a wide band gap semiconductor such as SiC instead of Si. However, at such high temperatures the SiO2 layer becomes less insulating as the ionic charges within the layer become mobile. Under these conditions the device acts as a true capacitor only when biased in depletion (e.g., for n-type SiC, the gate voltage is negative with regard to the back contact). In accumulation, it begins showing some D.C. conduction. However, as the conduction process in the SiO2 is different from that in the SiC, there will be a finite voltage (the barrier potential) at which there is onset of this forward conduction. This barrier potential will depend on the charges injected into the SiO2 insulator by the chemical processes at the gate. As changes in this barrier voltage are directly reflected as changes in the current/voltage D.C. characteristic in the forward direction, a sensor response can be obtained by measuring changes in this characteristic as a function of changes in the combustible concentration near the catalytic gate. However, the mobility of charges in SiO2 is still relatively low. Therefore, to obtain a reasonable forward current with reasonable applied voltage the thickness of this SiO2 layer has to be very small, leading to breakdown instabilities.
The present invention provides a combustible gas sensor consisting of a diode structure which includes a silicon carbide semiconductor substrate having a metal back contact juxtaposed thereto. An AlN layer is deposited onto the SiC semiconductor substrate and a thin porous catalytically active metal electrode xe2x80x9cgatexe2x80x9d is deposited on the AlN layer.
This diode structure allows the detection of combustibles, such as hydrocarbons and carbon monoxide in a gas flow in a wide concentration range with the adjustment of certain parameters, such as the oxygen concentration. This device is compatible with semiconductor electronic technology and is substantially more robust than devices requiring membrane support for low thermal mass as the micro-calorimeter. The device further can be operated in either a forward conduction D.C. or a reverse bias A.C. response mode, the former not requiring radio frequency techniques.
In the D.C. mode embodiment, there is provided a method of detecting combustibles in a gas stream by placing a forward biased diode sensor in contact with the gas stream, wherein the diode sensor comprises a silicon carbide semiconductor substrate; a metal back contact juxtaposed to the silicon carbide semiconductor substrate; an AlN layer deposited onto the silicon carbide semiconductor substrate; and a catalytically active metal xe2x80x9cgatexe2x80x9d electrode deposited on the AlN layer. The method further includes the step of applying a-constant forward current to the diode sensor and measuring changes in the forward voltage drop across the diode sensor as the response to changes of the combustible concentration in the gas stream. Alternatively, a constant forward bias may be applied across the sensor measuring changes in the forward current through the device as the response to changes of the combustible concentration in the gas stream.
In the A.C. mode, the present invention also provides a method of detecting combustibles in a gas stream, by placing the reverse biased diode sensor in contact with the gas stream, wherein the diode sensor comprises a silicon carbide semiconductor substrate; a metal back contact in mating engagement with the silicon carbide semiconductor substrate; an AlN layer deposited onto the SiC semiconductor substrate; and a catalytically active metal gate deposited on the AlN layer. The method next involves the step of measuring the capacitance of the diode sensor at frequencies between 10 and 3000 KHz . A constant reverse bias voltage is applied across the diode sensor. Lastly, the method requires the step of detecting a change in the capacitance in response to a change of the combustible concentration in the gas stream.